Need something stable into <8 ohm loads

[Jean]

It happened again last night, blew two channels. First time couple months back I blew a channel due to me shorting something out while adjusting bias. This time one channel blew after I played some music as loud as it could possibly get w/o hearing any distortion. Worked for a couple minutes and then FLASH, poof and its silent. This time it didn't take my speakers out with it, lucky me.

Clearly my speakers aren't the easiest things to play, my onkyo receiver also goes into protect mode when played at loud levels.

What would be a good amplifier , I am just sick of blowing output transistors and I don't want to risk blowing my drivers again. I have aleph 2 boards completed, just need matching output devices and then its just a chassis work. Is there something else out there that I can look at ?

thanks.

[Jennice]

Hi Jean,

First your "tag"...
Right lane is for passing slow-movers who THINK they're entitled to the left lane.

Secondly, your gear:
to get an idea of your "loud playing", what's your Onkyo rated at (8 ohms) ?

There could be three main categories for your problems.

1)
HF oscillations causing heat dissapation which you can't hear.

2)
Insufficient thermal compensation, causing thermal runaway in your output transistors

3)
Insufficient number of output transistors / insufficuient heat sinking

Option 1 is less likely, since your onkyo also cuts out with your speakers.

If you've ever monitored the temperatures on your heat sinks or transistor casings, you should be able to determine whether it's 2 or 3 which kills your amp.

What sort of power level was your last DIY rated (estimated) at (watt/ch), and how much heat sinking ('K /W) was applied?

It's important to remember that you have a thermal resistance between the transistor casing and the heat sink, and between the transistor's semi-conductor and the transistor housing (enclosure).

Jennice

[Jean]

Hi Jennice,

Originally posted by Jennice
Hi Jean,

First your "tag"...
Right lane is for passing slow-movers who THINK they're entitled to the left lane.


Secondly, your gear:
to get an idea of your "loud playing", what's your Onkyo rated at (8 ohms) ?

Yes, it is rated at 8 ohm.

There could be three main categories for your problems.

1)
HF oscillations causing heat dissapation which you can't hear.

I don't think its oscillating, I could leave it on for hours and there is 0 heat.

2)
Insufficient thermal compensation, causing thermal runaway in your output transistors

Amp was cold actually when I turned it on. I was just in the mood to play it at absolute maximum volume. It only took about 1 minute to blow both channels, heatsinks were warm around the output devices.

3)
Insufficient number of output transistors / insufficuient heat sinking

Hmm, then maybe I could just put another pair of output transistors here ?

Option 1 is less likely, since your onkyo also cuts out with your speakers.

If you've ever monitored the temperatures on your heat sinks or transistor casings, you should be able to determine whether it's 2 or 3 which kills your amp.

I haven't measured temperatures, but I can tell you that I played this amp for many hours at LOUD levels w/o anything blowing up. This was just a loud record and I pushed it to the absolute maximum.

What sort of power level was your last DIY rated (estimated) at (watt/ch), and how much heat sinking ('K /W) was applied?

Its a p3a amp, with 36volt rails with 21193/21194 pair. My speakers measured at 3.5ohm with a DMM, I have no way of plotting impedence.

It's important to remember that you have a thermal resistance between the transistor casing and the heat sink, and between the transistor's semi-conductor and the transistor housing (enclosure).

Jennice

[Jennice]

Are you trying to tell me you're pulling tough loads with an amp that has ONE output pair???

[Jean]

Yep, its one tough amp I just pushed it too much. Do you think adding a pair or two will work its magic ?

[MikeW]

I would double up on the output transistors. Look at Rod's article on safe operating area. Put some little heatsinks on the drivers too. Then crank it.
P.S. Thanks for the transformer. Its in a Zen 5.

[Jennice]

Hi Jean,

I guess we now know just how tough the amp is...

Adding additional output devices will surely ease the load on the components for safer operation (and maybe you want to consider additional heat sinking?)
However, some schematics easily allow for parallel output devices, others not as easily. You sound rather familiar with DIY'ing, so you should figure out. Otherwise let me know (if so, please provide a link to it).

Jennice

[hugobross]

This was just a loud record and I pushed it to the absolute maximum.

Have you ever considered a limiter at the input?
Probably you've pushed the amp into clipping, causing too much heat dissipation or HF oscillation. Maybe a clipping indicator is also a good tool for you!!
Anyway, double the amount of output transistors and place a simple limiter at the input.


Best regards,

HB.